7511
J. Phys. Chem. 1993,97, 7511-7515
Inductive and Steric Effects on the Structure of Isopropyl Acetate. Gas Electron Diffraction and ab Initio MO Investigations Hiroshi Takeuchi, Masami Sugino, Toru Egawa, and Shigehiro Konaka' Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan Received: February 16, 1993; In Final Form: April 29, I993
The molecular structure and conformation of isopropyl acetate, C(3)H3C( 1) [=0(2)]0(4)C(5)H[C(6)H3]C(7)H3, have been studied by gas electron diffraction combined with HF/4-21G calculations and vibrational spectroscopy. Diffraction data are consistent with the assumption that the C1-02 bond is eclipsed with the 04-C5 bond. At room tempeature the relative abundance of the conformer with the C104C5H dihedral angle, 9, of 35' is determined to be 94(9)%, and a small amount of a second conformer with a 9 value of 180' (Cl-04 trans to C5-H) may also be present. The structural parameters ( r g / Aand L,/deg) of the predominant conformer are: r ( C 1 4 2 ) = 1.203(5), r(C1-04) = 1.334(6), r(04-C5) = 1.438(6), (r(C5-C)) = 1.522(2), (r(C5-C)) -r(Cl-C3) = 0.010 (calculated), (r(C-H)) = 1.100(5),LO2ClC3 = 121.3(23),L02C104 = 124.1(8),LC104C5 = 119.0(11),L04C5Ct= 1O5.8(22),L04C5Cg= 109.7(34),LCCC = 113.5 (calculated), and (LCCHM.,) = 107.5(17), respectively. Parenthesized values are error estimates (3a). Symbols Ct and C, denote the terminal carbon atoms, which are trans and gauche to the C1-04 bond, respectively, and ( ) denotes average values. The differences between the structural parameters of the two conformers and the differences between structural parameters of similar types have been taken from theHF/4-21Gcalculations. The (O=)C-O bond length in CH3COOR (R = H, Me, Et and i-Pr) is found to decrease with the increasing size of the substituent, R. This is attributed to the electron-releasing inductive effect of alkyl groups. The steric effect of alkyl groups is seen in the C104C5 and 0 2 C l C 3 bond angles. Observed structural trends have been reproduced qualitatively by HF/4-21G* as well as HF/4-21G calculations.
02
Introduction
n
Recently we have investigated the molecular structure of ethyl acetate by gas electron diffraction (GED).' Comparingthe result with the structures of acetic acid2 and methyl acetate,' we found the following remarkable substitution effect on the structures of CH3COOR (R = H, Me and Et): the (O=)C-0 bond length tends to decrease in the order R = H (1.363(9) A), R = Me (1.360(6) A), and R = Et (1.345(3) A), although the difference between the first two is not significant. This effect can be attributed to the electron-releasing inductive effect of alkyl groups.' In order to examine the substitution effect on the structures of alkyl acetates in more detail, we have studied the molecular structure and the conformationalbehavior of isopropyl acetate (IPA) by GED. Figure 1 shows two possible conformationsof IPA. Riggs and Verma4 studied the conformation by NMR spectroscopy and suggested that the C1 conformer is much more stable than the C, conformer. Saunders et al.5 observed the far-IR spectrum in the liquid phase. George et a1.6 and Kn6zinger7 measured the temperature dependence of IR spectra. Only one conformer was detected by these spectroscopic studies. The determination of the molecular structure of IPA by GED alone is rather difficult because there are many inequivalent atom pairs. In the present study we obtained supplementary information from ab initio MO calculations at the HF/4-21G levels for the following reason. (1) The calculationsgive reliable values for the differences between bond lengths and angles of similar types? Thus such structural differences can be used as constraints in the data analysis of GED. (2) The force constants given by ab initio calculations can be used in the normal-coordinateanalysis. The force constants scaled so as to reproduceobserved vibrational frequenciesare expected to give reliable values of mean amplitudes and shrinkage corrections. The HF/4-21G calculations on acetic acid, methyl acetate, and ethyl acetate reproduced the abovementionedstructural trend only qualitatively.1 In order to compare experimental results 0022-3654/93/2097-75 11$04.00/0
0.4
H8
Figure 1. CI(upper) and C, (lower) conformers of isopropyl acetate.
with ab initio calculations with an improved basis set, HF/421G* calculations were also carried out for the most stable conformers of these compounds and IPA.
Experimental Section Thesampleof IPA, with a puritybetter than 99%,was obtained from Tokyo Chemical Industry Co.,Ltd. and used without further purification. An apparatusloequipped with an r3-sector was used to record the diffraction patterns of IPA on 8 X 8 in. Kodak electron image plates. Electron image plates were developed in Kodak D-19 (no dilution) for 12 min. Camera distance was 245.1 mm. The wavelength of incident electronswas determined from the diffraction patterns of carbon disulfide (r,(C=S) = 1.5570 recorded on two photographic plates. Other 0 1993 American Chemical Society
Takeuchi et al.
7512 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 experimental conditions are as follows: room temperature, 27 OC; electron wavelength, 0.06364 A; sample pressure, 20-24 Torr; background pressure, (3.2-3.4) X 10-6Torr; exposure time, 2224 s; beam current, 3.0 pA; uncertainty in the scale factor, 0.06%. Two photographic plates were selected for the data analysis of IPA. Optical densities were measured with a microphotometer of a double-beam autobalanced type. Five adjacent points with intervals of 100 pm were averaged and converted to total intensities. The total intensities leveled by using theoretical background intensities were averaged for two plates. Elastic and inelastic scattering factors were taken from refs 12 and 13, respectively. The IR spectrum in the vapor phase was measured at room temperature with a Bomem Model DA3.16 spectrometerby using a 10-cm cell with KBr windows. The far-IR and Raman spectra in the liquid phase were measured at room temperature with a DIGILAB Model FTS-65DF spectrometer and a Jasco Model R300S Raman spectrometer using a He-Ne laser (632.8 nm), respectively. The normal-coordinateanalysis of observed vibrational spectra is described later.
Theoretical Calculations Ab initio calculations were carried out by using the program GAUSSIAN 86.14 Except for the configurations of methyl groups, two dihedral angles about C1-04 and 0 4 4 5 bonds specify the conformation. The molecule was found to take an energy minimum at the 02C104C5 dihedral angle of Ool* as expected from the experimental data of acetic acid,* methyl acetate,3 and ethyl acetate.' Geometrical parameters including this dihedral angle were optimized for the pseudoconformers with C104C5H dihedral angles, 4, of 0,20,40,60,90, 120, 150, and 180°, at the HF/4-21G* level with default convergence criteria. The 4-21G basis set8used in this study is not the same as the basis set deposited in the GAUSSIAN program. A local energy minimum was found in the vicinity of 4 = 40°. The dihedral angle 4 was also optimized for the conformer corresponding to this energy minimum. It was found that there are two energy minima and that the C1 conformer at = 40.9O is more stable than the C, conformer with 4 = 180.0° by 1.4 kcal mol-1. Calculated relative energies of the other pseudoconformers are given in Table SI (supplementary material). From this energy difference, the population of the C1 conformer was calculated to be 95% at room temperature by assuming a Boltzmann distribution. The optimized geometries of the conformers in energy minima are given in Table I. Ab initio calculations at HF/4-21G* (4-21G plus d-functions with orbital exponent 0.8) level were performed for the most stable conformers of acetic acid and methyl, ethyl and isopropyl acetates. Optimized geometries are listed in Table 11. Cartesian force constants werecalculatedwith the GAUSSIAN 86 program at the HF/4-21G level. They were converted to the force constantsfor the local symmetry coordinateslisted in Table SI1(supplementary material). They were modified by using scale factors e:16 Scale factors were determined for the C1 conformer since it exclusively exists in the vapor phase as described later. The scale factorswith similar values for diagonal force constants or similarity in local symmetry coordinates were set to be equal in order to reduce the number of independent scale factors. In some other cases, however, scale factors were also assumed to be equal by referring to 4-21G scale factors of methyl acetate.) Scale factors were finally divided into 13 groups and they were simultaneously determined by least-squares fitting of vibrational frequencies. Scaled diagonal and off-diagonal force constants are listed in Tables SI1 and SI11 (supplementarymaterial), respectively. The scale factor for each group is given in Table SIV (supplementary
TABLE I: 4-216 Geometries of the Stable Conformers of hDrODYi Acetate. param
9
C S
c 1 4 2 C1-04 C546 C5-H8 ( C~-HMC)' C3C104 02C104 04C5C6 04C5H8 ( C5C6Hyc)' C6C5C7 C7CSH8 02C104C5 C104C5C6 C104CSH8 02ClC3H10 04CSC6H12 04C5C6H14 04CSC7H16
1.208 1.356 1.529 1.078 1.082 110.7 123.1 109.1 107.5 109.9 113.5 111.4 0.0 -78.6 40.9 -121.0 -177.1 -57.4 -58.9
1.208 1.354 1.532 1.078 1.081 110.2 124.2 110.7 101.6 109.9 113.5 109.9 0.0 63.3 180.0 -121.0 178.0 -62.0 -58.0
param
c.
CI
C143 1.507 1.475 04-43 1.525 CS-C7 ( c 3 - H ~ ~ ) ' 1.080 ( c 7 - H ~ ~ ) ' 1.082 126.2 02ClC3 119.1 C104C5 104.8 04CSC7 109.4 (ClC3H& ( C ~ C ~ H M . , ) '110.1 C6C5H8 110.2
1.508 1.477 1.532 1.080 1.081 125.6 122.4 110.7 109.4 109.9 109.9
c3c104c5 C104CSC7 02ClC3H9 02ClC3Hll 04C5C6H13 04C5C7H15 04CSC7H17
180.0 -63.3 0.0 121.0 58.0 -178.0 62.0
180.0 159.5 0.0 121.0 63.1 -179.2 60.8
a Bond lengths in A and bond angles and torsional angles in deg. Total energies of the Cl and C,conformers are -344.13686 and -344.13468 au, respectively. Average values.
*
TABLE Ik Calculated Geometries of the Most Stable Conformers of CHXOOR (HF/4-21G*P . R=H
R=Me
R=Etb
R=i-Pf
1.182 1.510 1.338
1.183 1.512 1.334 1.424
1.183 1.512 1.333 1.432 1.519
126.0 123.0
125.6 123.8 115.0
125.5 123.9 115.5 107.1
1.184 1.513 1.332 1.441 1.523 1.526 125.1 124.5 117.0 105.6 109.7 113.1
r(-) r((o=)C-C) r((O=)C-0) r(0-C) r(C-Ct)d r(C4l;)d LM-C L M - 0
LCQC LOCCtd LOCC,d LCtCC,d
Bond lengths in A and angles in deg. Total energies of the molecules are -227.39101 au (R = H), -266.36075 au (R = Me), -305.341 19 au (R = Et) and -344.32126 au (R = i-Pr). The trans conformer. The Cl conformer. The values of the dihedral angles, COCH, COCCt, and COCC,, are 39.5,157.7,and-80. lo,respectively. C G a n d C-C,denote the C-C bonds trans and gauche to the (O=)C-O bond, respectively.
material). In the case of methyl acetate? the scale factors corresponding to groups 1-2, 3, and 5 (see Table SIV) at the HF/4-21G level are 0.833,0.767, and 0.864, respectively, which are in close agreement with the values determined for IPA. Calculated frequencies are in good agreement with the observed ones as shown in Table SV (supplementary material).
Structural Analysis The following assumptions were made on the molecular structure of IPA. (1) The OCOC dihedral angle is Oo, and the isopropyl group takes two equilibrium configurations as predicted by the 4-21G calculations. (2) The structural differencesbetween the two conformersare given by the 4-21G calculations. (3) The differences between Cl-C3 and C5-C bond lengths are equal to the corresponding differences in the re distances estimatedfrom the re(4-21G)structure and empiricalcorrections, rg- re(4-21G) (0.002 A for r(Cl-C3) and -0.008 A for r(C5cy'). (4) The differencesin C-H bond lengthsand CCH bond angles are the same as those given by the 4-21G calculations. ( 5 ) The CCC bond angle is equal to the 4-21G value.
Structure of Isopropyl Acetate
The Journal of Physical Chemistry, Vol. 97,No. 29, 1993 7513
TABLE III: Mean Amplitudes ( I ) and Interatomic Distances (I.) for Isopropyl Acetate (A).
9 atom pair
lo"
C,
aroud
r.
IO"
nroud
r.
2.391 ' 0.064 0.069 3.722 0.070 0.081
1 1 1 2 3 2 2 2 2 2 3
1.202 1.512 1.331 2.419 3.004 2.363 2.246 2.793 2.993 2.386 3.743
0.137 0.141 1.091. 0.078 0.075
4 1
4.366 1.091
1.095 0.078 0.076 1*095 0.049
2.357 0.071 0.075
1 1 2
1.095 1.438 2.445
1.092' 2.543 1.097 1.094 1.096 1.097 1.095 1.095
1 1 2 1 1 1 1 1 1
1.526 1.092 2.500 1.097 1.096 1.092 1.097 1.096 1.092
l a
~~
C142 C1
